1. Field of the Invention
The invention relates to the field of micromachined gyroscopes, and in particular to inertial micromachined transducers for measurement of angular rotation rate of an object.
2. Description of the Prior Art
Even though an extensive variety of micromachined gyroscope designs and operation principles exists, almost all of the reported micromachined gyroscopes use vibrating mechanical elements to sense angular rate. The concept of utilizing vibrating elements to induce and detect Coriolis force involves no rotating parts that require bearings, and have been proven to be effectively implemented and batch fabricated in different micromachining processes.
The operation principle of the vast majority of all existing micromachined vibratory gyroscopes relies on the generation of a sinusoidal Coriolis force due to the combination of vibration of a proof-mass and an orthogonal angular-rate input. The proof mass is generally suspended above the substrate by a suspension system consisting of flexible beams. The overall dynamical system is typically a two degrees-of-freedom (two DOF) mass-spring-damper system, where the rotation-induced Coriolis force causes energy transfer to the sense-mode proportional to the angular rate input.
In most of the reported micromachined vibratory rate gyroscopes, the proof mass is driven into resonance in the drive direction by an external sinusoidal electrostatic or electromagnetic force. When the gyroscope is subjected to an angular rotation, a sinusoidal Coriolis force is induced in the direction orthogonal to the drive-mode oscillation at the driving frequency. Ideally, it is desired to utilize resonance in both the drive and the sense modes in order to attain the maximum possible response gain, and hence sensitivity. This is typically achieved by designing and electrostatically tuning the drive and sense resonant frequencies to match.
Alternatively, the sense-mode is designed to be slightly shifted from the drive-mode to improve robustness and thermal stability, while intentionally sacrificing gain and sensitivity. However, the limitations of the photolithography-based micromachining technologies define the upper-bound on the performance and robustness of micromachined gyroscopes. Conventional gyroscopes based on exact or close matching the drive and sense modes are extremely sensitive to variations in oscillatory system parameters that shift the natural frequencies and introduce quadrature errors, and require compensation by advanced control architectures.
Micromachined gyroscopes are projected to become a potential alternative to expensive and bulky conventional inertial sensors in the near future. High-performance gyroscopic sensors including precision fiber-optic gyroscopes, ring laser gyroscopes, and conventional rotating wheel gyroscopes are too expensive and too large for use in most emerging applications. With micromachining processes allowing mass-production of micro-mechanical systems on a chip together with their control and signal conditioning electronics, low-cost and micro-sized gyroscopes will provide high accuracy rotation measurements.
Moreover, advances in the fabrication techniques allow the detection and control electronics to be integrated on the same silicon chip together with the mechanical sensor elements. Thus, miniaturization of vibratory gyroscopes with innovative micro-fabrication processes and gyroscope designs is expected to become an attractive solution to current inertial sensing market needs, as well as open new market opportunities. With their dramatically reduced cost, size, and weight, MEMS gyroscopes potentially have a wide application spectrum in the aerospace industry, military, automotive industry and consumer electronics market. The automotive industry applications are diverse, including high performance navigation and guidance systems, ride stabilization, advanced automotive safety systems like yaw and tilt control, roll-over detection and prevention, and next generation airbag and anti-lock brake systems. A very wide range of consumer electronics applications include image stabilization in video cameras, virtual reality products, inertial pointing devices, and computer gaming industry. Miniaturization of gyroscopes also enable higher-end applications including micro-satellites, micro-robotics, and even implantable devices to cure vestibular disorders.
The tolerancing capabilities of the current photolithography processes and micro-fabrication techniques are inadequate compared to the requirements for production of high-performance inertial sensors. The resulting inherent imperfections in the mechanical structure significantly limits the performance, stability, and robustness of MEMS gyroscopes. Thus, fabrication and commercialization of high-performance and reliable MEMS gyroscopes that require picometer-scale displacement measurements of a vibratory mass have proven to be extremely challenging.
In the conventional micromachined rate gyroscopes, the mode-matching requirement renders the system response very sensitive to variations in system parameters due to fabrication imperfections and fluctuations in operating conditions. Inevitable fabrication imperfections affect both the geometry and the material properties of MEMS devices, and shift the drive and sense-mode resonant frequencies. The dynamical system characteristics are observed to deviate drastically from the designed values and also from device to device, due to slight variations is photolithography steps, etching processes, deposition conditions or residual stresses. Variations in the temperature of the structure also perturb the dynamical system parameters due to the temperature dependence of Young's Modulus and thermally induced localized stresses.
Extensive research has focused on design of symmetric suspensions and resonator systems that provide mode-matching and minimize temperature dependence. Various symmetric gyroscope designs based on enhancing performance by mode-matching have been reported. However, especially for lightly-damped devices, the requirement for mode-matching is well beyond fabrication tolerances; and none of the symmetric designs can provide the required degree of mode-matching without active tuning and feedback control under the presence of the mentioned perturbations. Also the gain is affected significantly by fluctuations in damping conditions, which makes the device very vulnerable to any possible vacuum leak in the package.
Fabrication imperfections also introduce anisoelasticities due to extremely small imbalances in the gyroscope suspension. This results in mechanical interference between the modes and undesired mode coupling often much larger than the Coriolis motion. In order to suppress coupled oscillation and drift, various devices have been reported employing independent suspension beams for the drive and sense modes. Consequently, the current state of the art micromachined gyroscopes require an order of magnitude improvement in performance, stability, and robustness. Fabrication imperfections and variations, and fluctuations in the ambient temperature or pressure during the operation time of these devices introduce significant errors, which have to be compensated by advanced control architectures.